In work supported by AI30084, we found that tissue damage in amebiasis arises from complex interactions between host and parasite, and that within different host environments E. histolytica is capable of inducing inflammation or programmed cell death. Using a severe combined immunodeficient mouse/human intestinal xenograft (SCID-HU-INT) model of intestinal amebiasis, which mimics human disease, we found that E. histolytica trophozoites induce human intestinal epithelial cells to produce inflammatory mediators in vivo. These mediators induce neutrophil influx into the intestine with resultant tissue damage. Blocking inflammation by inhibiting activation of the transcription factor NF-kappaB in intestinal epithelial cells reduces amebic damage to the intestine. Thus, parasite- enterocyte interactions play a key role in triggering the host inflammatory response, and inflammation plays a significant role in the tissue damage seen early in amebic infection. These findings contrast with those from our studies of amebic liver abscess using our murine model of disease. We found that there is limited inflammation in amebic infection of liver, and that neutrophils play a protective role in limiting abscess size. Strikingly, within amebic liver abscesses, hepatocytes are dying by apoptosis, with E. histolytica inducing programmed cell death by a Fas ligand- and TNF alpha receptor-independent pathway. The goal of this proposal is to dissect, at the molecular level, how E. histolytica induces host cell responses of inflammation or apoptosis, and to determine whether inhibiting those host responses, by targeting molecules in either the parasite or host, represents a viable approach to blocking disease. Our development of the SCID-HU-INT and murine amebic liver abscess models, and our ability to utilize transfection approaches for specifically inhibiting or augmenting expression of targeted amebic genes, makes these in vivo studies feasible for the first time. We will extend our analysis of the host response to amebic infection by identifying host genes whose expression is altered in response to E. histolytica infection of intestine or liver by screening human or murine cDNA microarrays. These studies will enable us to understand how ameba damage host tissue in vivo, and should provide new paradigms for the interactions between intestinal protozoans and their human hosts.